Ophthalmic lenses serve basically three purposes:
1. CORRECTION OF VISION DEFECTS;
2. PROTECTION AGAINST MECHANICAL HAZARDS TO THE EYE;
3. PROTECTION AGAINST RADIATION.
The first purpose is accomplished by providing transparent articles in the form of lenses with well defined refractive power. The second purpose is accomplished by either (1) using impact resistant materials for lens construction or (2) judicious treatment of glass lenses to provide additional strength. Such methods include, for example, the well known and widely practiced procedure of heating lenses above the strain point and rapidly quenching them in either an air stream or a liquid. An example of the latter process is described in U.S. Pat. No. 3,768,992. A second method of increasing strength is to provide for a compression envelope over the surface of the lens. Such methods are described, for example in U.S. Pat. No. 3,790,260 and in pending patent application file number AO-2366 Ser. No. 157,481, filed June 28, 1971, owned by the assignee of this application.
Protection against such radiation as ultraviolet light, intense visible light or the infrared can be achieved by adding colorants to the glass batch. Examples for such colorants can be found in the book COLOURED GLASSES by W. A. Weyl, Soc. Glass Technol., Sheffield, England, 1967. In the case of ophthalmic lenses made from high polymers, colorants can be added to the material itself as has been done in some commercially available sunglasses for many years. It is also possible to tint colorless plastic lenses by exposing them to a surface dyeing process. An example of such a lenses is available in commerce under the trade name "Tintolite" sold by American Optical Corporation of Southbridge, Massachusetts. Clear glass lenses can also be tinted by exposing them to a so-called "staining process" of which many examples can be found in the above mentioned book by W. A. Weyl. A specific example of such a staining process applied to glass lenses is described in the pending patent application file number AO-2958 Ser. No. 610,608 filed Sept. 5, 1975 owned by the assignee of this application. A wide variety of permanently colored lenses useful for ophthalmic application are available in commerce and sold under such trade names as "Truecolor", "Cruxite", "Cosmetan", "Calobar", all manufactured by American Optical Corporation.
Permanently colored or dyed ophthalmic lenses have a disadvantage of retaining low transmission of light when the wearer of such lenses is exposed to low levels of illumination i.e., is in a more or less dark environment. A typical example of such an environment would be sunglasses worn during nighttime driving which can, of course, be hazardous. This disadvantage can be overcome, to a certain extent, by the many varieties of phototropic or photochromic glass or plastic lenses available in commerce today. Phototropic ophthalmic lenses have been described for example in U.S. Pat. No. 3,197,296. The lenses described therein in essence are transparent to visible radiation but will darken upon exposure to actinic radiation, having a transmission to visible radiation of about 45% of the original transmissivity. The reduction in transmissivity of such lenses is reversible with a half-fading time of not more than five minutes. Some other prior art of which we are aware includes: the ophthalmic lenses described in U.S. Pat. No. 3,197,296 utilizing a specific glass composition falling within the following ranges, on a weight percent basis: 48 to 58 SiO.sub.2, 6 to 10 Al.sub.2 O.sub.3, 15 to 22 B.sub.2 O.sub.3, 0.8 to 2.0 Na.sub.2 O, 2.4 to 3.1 Li.sub.2 O, 0 to 4 K.sub.2 O, the total Li.sub.2 O plus Na.sub.2 O plus K.sub.2 O being 3.2 to 7.2, 4.5 to 5.3 PbO, 3 to 9 BaO, 0 to 7.2 ZrO.sub.2, 0.15 to 0.6 Ag, 0.01 to 0.02 CuO, 0.3 to 1.2 Cl, 0 to 1.0 Br, 0 to 1.0 I, and 0 to 1.2 F; numerous phototropic or photochromic glasses described for example in U.S. Pat. Nos. 3,208,860; 3,548,060; 3,594,198; 3,617,316; 3,703,388; 3,765,913; 3,795,523; 3,833,511; 3,834,912; British Pat. No. 1,275,019; German Pat. No. 2,230,506; and German Auslegeschrift No. 2,256,775.
In addition to the above mentioned patents on photochromic glasses, all containing silver halide particles uniformly dispersed throughout the volume of an article made from them, we know Chance-Pilkington Optical Glass Company, England, is marketing a phototropic phospho-silicate glass under the trade name "Reactolite". No reference describing compositional details and methods of making this glass has been found by us in the patent literature to date.
Photochromic glasses sensitized by silver halides are also know to us as described in general in the following articles:
W. h. armistead and S. D. Stookey: "Photochromic Silicate Glasses Sensitized by Silver Halides", SCIENCE, Vol. 144 (1964) pp. 150-154; PA1 G. gliemeroth and K. H. Mader: "Phototropic Glass", Angew. Chem. Internat. Edit., Vol. 9 (1970) pp. 434-445; PA1 A. v. dotsenko et al.: "A Study of the Effect of Copper Ions on the Relaxation Properties of Photochromic Glasses", Sov. J. Opt. Technol., Vol. 41 (1974) pp.395-397; PA1 R. j. araujo: "Photochromic Glasses", Chapter 8 of the book PHOTOCHROMISM edited by G. H. Brown, Willey Interscience, New York (1971) pp. 667-686; PA1 H. bach and G. Gliemeroth: "Phase Separation in Phototropic Silver-Halide-Containing Glasses", J. Amer. Cer. Soc. (1971) pp. 43-44.
All these prior art glasses seem to have in common that:
1. the ingredients producing the photochromic or phototropic behavior are silver halide particles uniformly dispersed in a glass matrix; and
2. articles made from these glasses must be exposed to a well defined heat treatment to develop photochromic or phototropic behavior.
The glasses also appear to differ from each other in the compositions of the base glasses which serve as carriers for the phototropic or photochromic centers. In general terms a phototropic glass article is described in U.S. Pat. No. 3,208,860. This patent discloses a phototropic article comprising a silicate glass body having in at least a portion thereof microcrystals of at least one silver halide selected from the group consisting of silver chloride, silver bromide, and silver iodide, the concentration of said crystals in said portion being at least 0.005% by volume.
In addition to glass articles containing silver halide crystals dispersed throughout their entire volume, we know it has also been proposed to prepare photochromic glass lenses by diffusing silver ions into a surface layer of a base glass containing quantities of halide ions and subsequently exposing these articles to a specific heat treatment. Such an article and the process of making it is described for example in U.S. Pat. No. 3,419,370.
It also has been proposed to prepare photochromic articles by applying phototropic coatings on substrates such as glass or plastic. Such articles are described for example in U.S. Pat. No. 3,875,321 and in a paper by G. Gliemeroth at the 75th Annual Meeting of the American Ceramic Society, Cincinnati, Ohio, May 2, 1973, which was subsequently published in the Journal of the American Ceramic Society (1974) pages 332-335 under the title "Reversible Optical Density Changes in Composite Layers".
Photochromic or phototropic lenses of the kind described above overcome to a certain extent the above-noted disadvantages of permanently tinted lenses. Because of the reversibility of the photochromic effect they assume a low transmissivity if exposed to ultraviolet or blue light but resume high transmissivity in an environment where low illumination levels of activating radiation prevail. Glass lenses have the known advantage over plastic photochromic lenses of more scratch resistance and they do not appear to lose photochromic properties during extended wear due to the degradation of active ingredients.
All photochromic or phototropic lenses presently known to us have the disadvantage that recovery of high transmissivity takes several minutes. This has been noticed with discomfort and dislike by wearers under such conditions as driving an automobile where low levels of illumination exist inside the car and high levels of illumination outside of the vehicle. It obviously is desirable to reduce the light intensity to a driver's eyes while he or she is observing road and traffic conditions, but simultaneously he or she must be permitted to observe clearly information presented by instruments on the vehicle dashboard where a low level of illumination normally exists. Indeed, it can be dangerous to prevent this. Other instances which illustrate the problem are found in occupations where sudden changes in the level of illumination from bright to dim occur either (1) by rapid changes in the intensity of the light source of (2) by movement of the wearer of the spectacles from an area of high illumination to a darker environment. Similar disadvantages have been observed by wearers of permanently colored lenses.
Some of the above disadvantages have been overcome by use of eyeglasses with a continuous variation of transmissivity from low at the top of the lens to high over the lower portion of the lens. Lenses with such a permanent gradient in degree of color or tint now are available in commerce. We believe the lenses are being prepared by differentially dyeing plastic lenses or by applying a gradiated colored coating over glass lenses by vacuum deposition of absorbing materials. With plastic lenses the color gradient can be achieved by continuously or progressively changing the concentration of dye absorbed by the lens. For example, a high concentration prevails at the top and a low concentration at the bottom of a lens.
In U.S. Pat. No. 3,419,370 we find the statement on representation that a gradient in photochromic behavior across a glass body is attainable by varying the time and/or temperature at which different portions of the glass body are exposed to an ion exchange medium. According to this patent the ion exchange bath contains, in all instances, silver ions (see Table 2 of the patent). The gradient in photochromic properties is achieved by causing or allowing different concentrations of silver ions to diffuse into the glass. The patent teaches, in our opinion, that the glasses described cannot be made photochromic or phototropic without having been exposed to the diffusion process in the silver containing ion exchange bath prior to the heat treatment required to develop phototropic or photochromic behavior. The glasses do not contain any silver ions in the base composition. The patent does not make any specific reference or teaching that we can find of a photochromic gradient over ophthalmic lenses.
In our opinion, the state of the art of making ophthalmic lenses uniformly phototropic or photochromic throughout their entire volume can be summarized as follows:
1. Glasses of the types listed in Table I hereafter are melted following procedures known to those skilled in the art of glass making.
2. Lens blanks are made of these glasses by known methods such as pressing or casting.
3. These articles are exposed to a controlled heat treatment to develop silver halide particles of linear dimensions d falling essentially within the range 5 &lt; d &lt; 50 nm. The lower limit is required to produce photochromic or phototropic behavior, the upper limit to avoid light scattering unacceptable in ophthalmic products. The total concentration of these silver halide particles which are dispersed uniformly throughout the glass article should be at least 0.005 vol. %.
In our opinion, the state of the art of making glass articles with a gradient in photochromic or phototropic behavior as deduced from U.S. Pat. No. 3,419,370 can be summarized as follows:
1. A base glass having a composition in essence in the general system Alk. Oxide -- Al.sub.2 O.sub.3 -- B.sub.2 O.sub.3 -- SiO.sub.2, with addition of halides to the batch, is melted under conditions that allow retention of a sufficient quantity of halides.
2. Lens blanks are made from the glasses by known methods such as pressing or casting.
3. Finished lenses are made from the blanks by grinding and polishing.
4. The finished lenses are exposed to a source of silver ions at elevated temperature in such a fashion that in those parts of the lens where a high degree of phototropic of photochromic behavior is desired the silver concentration is higher than in those parts where a low degree of phototropic or photochromic behavior is desired.
5. The thus treated lenses are exposed to a carefully controlled heat treatment to grow silver halide crystals to a size required for photochromic or phototropic behavior, but not exceeding linear dimensions of 50 nm to avoid the light scattering unacceptable in ophthalmic lenses.